This circuit will turn on an AC Bulb when the sun sets, the working of the circuit is simple and the heart of this circuit is an LDR(Light Dependent Resistor) that is wired as a light sensor thus when the ambient light falls on the LDR the circuit will not work and when the ambient light fails to fall on the (when the sun sets) LDR the circuit will turn on a 230V ac bulb or CFL
circuit diagram of automatic lamp
how it works?
A TRIAC is used here instead of a relay, a triac can switch high voltage AC. The voltage to trigger the triac gets from the emitter of T1 so since resistance of the LDR is low as it is in day light the T1 will not get sufficient positive voltage to trigger the triac but in darkness the resistance of LDR is very low this causes to turn the T1 on so the triac will get triggered and the bulb will glow. D1 and R1 are used to rectify AC and lower the voltage, C1,R1 used to pass 6 VDC for T1. the R3 can be set to a desired level to turn the light on as the level of sunlight.
Pinout of triac
this circuit is not isolated from mains! Touching any parts of the circuit while connected to mains may cause to pass high voltage through your body and may kill you! If you are a newbie in electronics then don’t build this circuit!